White Paper for the design of Tailored of Plunger Valves.
Dr. Raúl Trujillo Álvarez
16 February, 2022
Abstract
The scope of this With paper is to provide guidelines for designing the Inherent Control Valve Flow Characteristics in such a way as to allow linear Installed Flow Characteristics.1 Scope
The inherent and installed flow characteristic, the relationship between flow coefficient and valve stroke, has been a subject of misunderstandings and endless debate with water transport and distribution systems operators.
The control valves must be analyzed from the perspective of their inherent characteristics and their system behavior (Installed characteristics). The “inherent flow characteristic” is the relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to-rated travel with a constant pressure drop across the valve. The Inherent flow characteristics are determined under laboratory conditions. But, what interests hydraulic design engineers, control engineers, and automation engineers are the installed flow characteristics. “The Installed flow characteristics” include both the valve and pipeline effects. In other words, One crucial requirement in selecting the proper control valve is ensuring that the valve has the properly installed flow characteristic. The installed flow characteristic usually differs from the inherent due to changes in pressure drop: as flow increases, there is less pressure drop available across the valve. It’s generally desirable to have a linear installed characteristic.
2 Basic definitions
Basic terminology used herein is based on definitions stated in “Control Valve Terminology” [2] or applicable IEC standards.
Flow coefficient: Flow coefficient is a constant (\(K_v\)) related to the geometry of a valve plus cylinder (obturator) for a given valve opening that can be used to predict flow rate; see ANSI/ISA-75.01.01 (IEC 60534-2-1 Mod)-2007, “Flow Equations for Sizing Control Valves,” [1] and ANSI/ISA-75.02.01-2008, “Control Valve Capacity Test Procedures” [3].
Inherent flow characteristic: The Inherent flow characteristic is the relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to-rated travel with a constant pressure drop across the valve [4]. The Inherent flow characteristics are determined under laboratory conditions by testing the valve flow versus valve position or travel using a constant differential pressure drop across the valve through the test. Manufacturers publish the inherent flow characteristics for each control valve plus the cylinder. The Inherent flow characteristics are standardized for a fixed pressure drop as defined using the valve flow coefficient value \(K_v\).
Inherent flow characteristic curves: Control valves can be mounted with different cylinders in such a way as to present different performance curves for the percentage of rated \(K_{v_{max}}\) versus the percent of rated travel of the cylinder. There are three basic characteristics: Quick opening; Linear and Equal percentage.
Relative flow coefficient (\(\phi\)): The relative flow coefficient is the flow coefficient (\(K_{v_{max}}\)) ratio at a stated full open to the flow coefficient (\(K_{v}\)) at rated travel [4].
3 Example A
It is a simple water transport system between two reservoirs, which are \(500 \; meters\) apart, and the elevation of the deposits is \(100\) and \(5 \; meters\), respectively. Before the entrance to the second tank, a control valve has been installed (see Flow Schematic).
Flow Schematic
Without the control system, the maximum flow possible the line gives and losses due to fittings and pipe are \(370.53\;l/s\). If a “Standard” plunger control valve with a flat portable cylinder is installed in the system, the maximum flow that will circulate through the transport systems will be \(369.18\; l/s\) -with a Zeta \((\zeta)\) of \(\zeta_{100\%}=3.8\) for the fully open valve and an additional gate valve with a Zeta of \(\zeta_{gate}=0.233\)
| Kv and Zeta Value | ||
|---|---|---|
| depending on valve position | ||
| Valve Position | Kv (l/s) | Zv (-) |
| 10% | 1.4 | 257,537.8 |
| 15% | 4.5 | 23,330.4 |
| 20% | 10.6 | 4,307.8 |
| 25% | 20.2 | 1,184.6 |
| 30% | 33.8 | 422.4 |
| 35% | 51.5 | 181.5 |
| 40% | 73.2 | 89.9 |
| 45% | 98.2 | 49.9 |
| 50% | 125.8 | 30.4 |
| 55% | 155.0 | 20.0 |
| 60% | 184.9 | 14.1 |
| 65% | 214.5 | 10.5 |
| 70% | 243.2 | 8.1 |
| 75% | 270.4 | 6.6 |
| 80% | 295.8 | 5.5 |
| 85% | 319.1 | 4.7 |
| 90% | 340.4 | 4.2 |
| 95% | 359.6 | 3.7 |
| 100% | 376.8 | 3.4 |